1. Field of the Invention
The present invention relates to low power pulsed anode magnetrons used to provide microwave energy, and more particularly, to a method for improving the output spectrum quality of the magnetrons.
2. Description of the Related Art
Low power pulsed anode magnetrons are commonly used to generate RF energy for assorted microwave applications such as airborne weather radar. The magnetrons commonly have a cylindrically shaped cathode centrally disposed a fixed distance from a plurality of radially extending anode vanes. The space between the cathode surface and the anode vane tips provides an interaction region, and a potential is applied between the cathode and the anode, forming an electric field in the interaction region. A magnetic field is provided perpendicular to the electric field and is directed to the interaction region by polepieces which adjoin permanent magnets. An internal heater is provided below the surface of the cathode, and by heating the cathode electrons are emitted thermionically. Electrons emitted from the cathode surface are caused to orbit around the cathode in the interaction region due to the magnetic field, during which they interact with an RF wave moving on the anode vane structure. The electrons give off energy to the moving RF wave, thus producing a high power microwave output signal.
Traditionally, weather radar systems were primarily directed towards identifying and localizing areas of increased density, such as clouds or other aircraft. In such applications, spectral control is less critical than overall output power. However, modern radar systems have placed increased emphasis on identifying slight changes in air pressure and utilize doppler effects to obtain greater detailed information. For example, wind shear can be identified through measurements of instantaneous changes of air pressure. To make these measurements, the radar system must detect very small frequency changes of the radar return signal. These operational demands have required that there be tighter control over the output frequency spectrum of the magnetrons than has been previously required.
Most commercial pulsed anode magnetrons suffer from two related problems which tend to degrade the consistency of the output frequency spectrum. A first problem experienced is that of undesired side lobes. A side lobe comprises a secondary rise in amplitude at a peripheral portion of the output spectrum, which essentially increases the bandwidth of the spectrum. The side lobe draws power away from the usable spectrum, thus wasting a portion of the output power of the magnetron. Moreover, by increasing the spectral width, it is increasingly difficult to detect minor frequency changes in the radar return signal.
A secondary problem facing commercial pulsed anode magnetrons is that of "twinning." The twinning phenomenon comprises the formation of a twin output signal, which duplicates a portion of the spectrum. In some cases, the problems do not surface until after the magnetrons have been deployed in operational radar units. The distorted signal can result in false readings by the operator of the radar system, which detects a phantom frequency shift caused by the presence of the twin signal. Output spectrums exhibiting the twinning phenomenon and the side lobes phenomenon are shown graphically in FIGS. 1 and 2, respectively.
Thus, there is a need to provide a low power pulsed anode magnetron having improved spectral quality and performance, without the problems of side lobes and twinning. In addition, it is further desirable to provide a method for improving the spectral quality of a magnetron both during and after assembly.